Taiwan's food safety watchdogs have no way yet to prove whether Chinese cabbage treated with a cancer-causing chemical is slipping in through Vietnam — bad news for shoppers who want to know where their vegetables came from.

• Reports out of China say cabbage there was kept fresh with formaldehyde, a chemical known to cause cancer.
• Taiwan bans Chinese cabbage outright, but there is wide suspicion that Chinese cabbage and baby bok choy are being routed through Vietnam and relabeled.
• The agriculture ministry admits it cannot yet identify where cabbage was grown, because it has not collected enough vegetable samples from exporting countries to build a reference database.
• Similar testing already works for shiitake mushrooms and tea, so the gap is specific to cabbage and related greens.
• Border checks stay split across customs for origin, the food and drug agency for safety and pesticide residues, and quarantine officials for plants.

Outlook: Until the sample database is built, officials are leaning on advice rather than testing — urging people to buy local, in-season vegetables with traceability or organic certification.